1. Field of the Invention:
The present invention relates to an automatically lockable slider for a slide fastener and more particularly a slider having a locking prong spring-biased to engage with a coupling head of a coupling element at an intermeshing region in its Y-shaped guide channel.
2. Prior Art:
A typical automatic lock slider for a slide fastener includes a pull tab and a locking prong operatively associated therewith. The locking prong is normally urged downwardly into the path of coupling elements in the slider, and can be retracted out of the path by lifting the pull tab on the slider.
As is often the case with slide fasteners having such sliders and used on a rigid garment fabric, they are opened by grabbing the garment fabric and spreading the fastener stringers forcibly apart instead of manipulating the pull tab on the slider. A similar scene is commonly witnessed at the site of sewing slide fasteners to the rigid garment fabric, namely a denim.
Japanese Utility Model Laid-Open Publication No. 51-126703 discloses a lockable slider having lock and ratchet functions. To effect the ratchet function, a cavity is defined in the bottom surface of the guide channel in confrontation with the locking prong so as to allow the coupling element to tilt and sink therein to pass underneath the prong when the fastener stringers are forcibly spread apart.
With this type of slider, the maximum allowable lock strength of the locking prong is defined by a critical pressure at which the coupling elements begin to shift out of position on the fastener or otherwise sustain damage. This means that the coupling element does not initiate tilting until it is engaged by the locking prong with considerable force nearly reaching the critical pressure. Under the influence of such pressure, the locking prong is susceptible to abrasion, which in turn makes the slider malfunction or be unlockable. Another problem with this prior slider is that it is difficult to attain the necessary dimensional precision in the finish of its parts.